In LTE (Long Term Evolution) and a successor system of LTE (e.g., also called LTE-A (LTE-Advanced), FRA (Future Radio Access), 4G, etc.), a D2D (Device to Device) technology that performs direct communications between user terminals without having intervention by a radio base station has been studied (e.g., Non-Patent Document 1).
The D2D technology may be able to reduce the traffic between the user apparatuses and the base station and to enable communications between the user apparatuses even when the base station is no longer able to provide communication services at the time of disaster or the like.
The D2D technology is roughly divided into a D2D discovery (also referred to as D2D discovery, D2D direction) for finding other communicative user terminals and a D2D communication (also referred to as a D2D direct communication, D2D communication, an inter-terminal direct communication, etc.) for performing direct communications between the terminals. In the following, when the D2D communication, the D2D discovery, etc., are not particularly distinguished, they are simply called “D2D”. Further, a signal transmitted and received by the D2D is called a D2D signal.
In 3GPP (3rd Generation Partnership Project), it is being studied to implement V2X by extending the D2D function. Note that the V2X is a part of ITS (Intelligent Transport Systems), and is a generic name of a V2V (Vehicle to Vehicle) indicating a communication mode executed between vehicles, a V2I (Vehicle to Infrastructure: roadside-to-vehicle communication) indicating a communication mode executed between a vehicle and a road-side unit (RSU: Road-Side Unit) installed at a roadside, a V2N (Vehicle to Nomadic device: device-to-vehicle communication) indicating form a communication mode executed between a vehicle and a mobile terminal of a driver, and a V2P (Vehicle to Pedestrian: pedestrian communication) indicating a communication mode executed between the vehicle and the mobile terminal of a pedestrian, as illustrated in FIG. 1.